1. Field of the Invention
The present invention relates to a transreflective liquid crystal display (LCD), and more particularly, to an RGBW transreflective LCD with improved image quality.
2. Description of the Related Art
LCDs are among the most widely used flat panel display devices. Generally, an LCD includes a pair of panels each having an electrode on inner surface, and a dielectric anisotropic liquid crystal layer interposed between the panels. In the LCD, the variation of the voltage difference between the field generating electrodes, e.g., the variation in the strength of an electric field generated by the electrodes, changes the transmittance of the light passing through the LCD, and thus desired images are obtained by controlling the voltage difference between the electrodes.
Depending on the type of the light source used for image display, an LCD may be classified as one of the following three types: a transmissive LCD, a reflective LCD, and a transreflective LCD. The transmissive LCD utilizes a backlight module as the light source, and the reflective LCD utilizes ambient light to illuminate the pixels from a front side. The transreflective LCD combines the transmissive and the reflective characteristics. Under medium light conditions such as an indoor environment or under complete darkness conditions, transreflective LCDs operate in the transmissive mode, while under bright conditions such as in an outdoor environment, they operate in the reflective mode.
Recently, an RGBW technology, in which a white subpixel is added to a set of red, green and blue subpixels, has been developed to improve the brightness of LCD panels. This technology can further enhance the resolution of an LCD by a rendering method. In the subpixel rendering method, red, green, blue and white subpixels are individually controlled. When a specific subpixel is displayed, the subpixels adjacent thereto are displayed along with the specific subpixel, so that a pixel is represented as the brightness divided by the specific subpixel and the adjacent displayed subpixels. With this method, more specific expressions of slant lines or curved lines become more possible, improving the resolution.
FIG. 1 is a sectional view of a conventional transflective LCD, which helps to illustrate the operation of such an LCD. As shown in FIG. 1, the LCD 10 includes a lower substrate 100, an upper substrate 160 and a liquid crystal layer 130 interposed therebetween. The upper substrate 160 has a common electrode 140 and a color filter 150 formed thereon. The color filter 150 includes red (R), green (G), blue (B) and white (W) regions 151-154. The lower substrate 100 has an insulating layer 110 and a pixel electrode 120 formed thereon. The pixel electrode 120 has an opaque portion 124 capable of reflecting ambient light and a transparent portion 122 capable of transmitting light from a backlight module 190 disposed at the exterior of the low substrate 100. The liquid crystal layer 130 is interposed between the low substrate 100 and the upper substrate 160. Therefore, the transreflective LCD 10 can display in both reflective and transmissive modes.
The R, G, B and W regions 151-154 respectively provide red, green, blue and white subpixels 11-14 to display different colors. Furthermore, the transmittance of the uncolored region 154 is above 0.9 (or deemed as 1), and that of each of the R, G, and B regions 151-153 is around ⅓. The B region 153 absorbs ⅔ of the ambient light 283 when the ambient light 283 passes through the B region 153. In particular, the B region 153 further absorbs ⅔ of the reflective light 283′ when the reflective light 283′ passes through the B region 153. That is, the ratio of the reflective light 283′ to the incident ambient light 283 is 1:9. However, the transmitting light 273 from the backlight module 190 passes through the B region 153 just one time so that the ratio of the transmitting light 273 to its incident light is 1:3. Similarly, regarding to the W region 154, the ratio of the reflective light 284′ to the incident ambient light 284 is above 81:100, and the ratio of the transmitting light 274 to its incident light is above 9:10.
Apparently, the percentages of effective output light of the white subpixel 14 and the subpixels 11-13 are inconsistent in both reflective and transmissive modes. Such an inconsistency gets worse, when the sub-pixel rendering technology is applied to the LCD 10. This leads to different chrominance in reflective regions and transmissive regions, decreasing display quality.